Method for the production of a sheet metal part by forming

ABSTRACT

A process of forming a sheet of metal having varying material thickness corresponding to selected strength and/or stiffness requirements generally consisting of varying the degree of heat applied during drawing of the metal to correspondingly vary the elongation coefficient of the sheet across the surface thereof.

BACKGROUND OF THE INVENTION

The invention relates to a process for the production of a shaped sheetmetal piece with different material thicknesses according to strengthand stiffness requirements by deep-drawing.

The generic production process is used in particular for lightweightconstruction and especially in the production of motor vehicle bodies.While in the past it was conventional to design a shaped sheet metalpiece in its thickness according to the area/section of highestmechanical requirements, in the meantime there has been a transition todifferentiation with respect to material thicknesses relative to thelocally different stiffness requirements.

A corresponding process is described for example in DE 43 07 563 C2. Thepatent explains a process for the production of a sheet metal structuralpart which has a multiple sheet structure consisting of a base sheet andin places a stiffening sheet or several stiffening sheets joined to it,the base sheet and the stiffening sheet or stiffening sheets beingjointly deep-drawn. Here the stiffening sheet or stiffening sheets is orare at least partially attached to the base sheet and are permanentlyjoined to the base sheet after deep-drawing.

One corresponding procedure is described in patent application DE 42 28396 A1, in which in addition to a partial increase in stiffness afurther objective is to reduce the oscillatory mass of flat or slightlydeformed areas of the sheet metal piece and thus to increase theeigenfrequencies.

The aforementioned prior art is subject to the disadvantage that such aprocedure requires relatively high expenditures with respect toproduction and logistics.

EP 0 486 093 B1 describes a process in which the reinforcement ofpartial areas of a shaped piece is done by means of a reinforcingstructure element which is formed separately from a plate body to becovered and which is only subsequently joined to the likewise completelyformed plate body. In this procedure the forming process is complicatedand expensive.

For the sake of completeness reference is made to DE 41 04 256 A1.There, using in particular the example of body parts for passenger carsand trucks, it is explained how the highly loaded local areas (hingeseats, lock reinforcements, attachment areas for sleepers and otherbearing parts) can be effectively reinforced. As a result, in thisprocess shaped pieces are produced which have also become known asso-called “tailored blanks” (see in this regard also VDI-Reports[Association of German engineers] No. 1002, 1993, pages 45-51). In thelatter citation it is shown especially using the example of inside doorsheet metal how sufficient stiffness can be achieved by larger sheetmetal thicknesses in the area of the hinge and lock attachment. Areduction in weight results in the thin sheet metal placed betweenthicker sheets. It is disadvantageous in these sheets that they canessentially only be used for shaped pieces which are not visible on thefinished product. The two aforementioned publications are explicitlygeared to shaped bodies which are either themselves an inner part of acombination of parts or which are reinforced by separate shaped pieceson one partial inner surface with disturbance of the outer surface.

The object of the invention is to devise a further process for theproduction of a shaped sheet metal piece with different materialthicknesses according to strength and stiffness requirements which canbe executed economically and without problems with respect to theforming process.

The approach as claimed in the invention is shown in the processfeatures according to the characterizing part of claim 1. A shaped sheetmetal piece produced in this way requires only relatively low additionalproduction and logistic expenditures. The shaped sheet metal pieces arecharacterized by high surface quality and smooth transitions between theareas of differing material thicknesses.

Heating in the area of the forming for deep-drawing of steel sheets,especially austenitic steel sheet, is inherently known from DE 23 32 287B2. In the area of force transfer on the other hand cooling takes place.Heat treatment is generally used to configure the deep-drawing processsuch that austenitic steel sheets can be formed. Heat treatment whichvaries over the surface of the sheet blank with the object of thisinvention, i.e., to obtain material thicknesses which differ accordingto strength requirements, does not take place.

Furthermore, DE 44 25 033 A1 discloses a process and a device forcompression forming of workpieces, a workpiece being clamped in aclamping device and being formed by at least one compression tool. Inparticular there is a laser beam means which causes the workpiece to beexposed to the laser beam and heated in order to reduce flow stress andimprove the capacity for deformation. The forming temperature can beadapted to different materials and can be controlled. In this way localheating of the workpiece can take place in the areas of high degrees offorming. In various embodiments it is also possible to reduce the wallthickness of the workpiece without detailing why this reduction isdesirable.

DE 43 16 829 A1 furthermore describes a process for material workingwith diode radiation, in which the beam profile can be matched to themachining process. Possible applications include: forming and bending ofa workpiece, laser beam flame cutting, welding of workpieces, removingimpurities or coatings from workpieces, local heating for support ofmetal cutting on workpieces and soldering of workpieces.

SUMMARY OF THE INVENTION

By using high strength, higher strength and extremely high strengthsteel sheet for example in motor vehicle body construction the weight ofparts can be reduced. But since these steels have only limitedworkability, their use is often precluded in certain areas of the partto be deep-drawn based on the required degree of forming dictated byfunction. As claimed in the invention, this defect is eliminated and theweight of parts is further reduced by different material thicknesses bythe fact that the flow limit is locally reduced via locally raising thetemperature before or during forming, especially deep-drawing, and thestrain-hardening and forming capacity is changed. This can result in thefact that in the areas in which high strengths/stiffnesses are requiredfor reasons of function, only little or no reduction of materialthickness occurs during forming, while on the other hand in the areas inwhich little or no strength/stiffness requirements are imposed, thesheet metal thickness can be reduced relatively dramatically duringforming, i.e., to a technically allowable degree.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a is a perspective view of a sheet metal blank of a uniformthickness, illustrating different zones thereof having been heateddifferently;

FIG. 1b is a vertical cross sectional view of the blank shown in FIG. 1aafter the blank has been plastically deformed to provide zones ofdifferent thicknesses formed in accordance with the present invention;and

FIGS. 2a and 2 b illustrates different yield strengths utilizingdifferent strain hardening treatment with respect to the blanks shown inFIGS. 1a and 1 b, following the process of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Depending on the sheet metal material used, the sheet metal thicknessand geometry of parts, a different flow behavior can be achieved, forexample by the versions listed below.

Version A:

Local heating takes place after one of the last rolling steps; thisyields coil sheets or single rolled sheets which have a deformationbehavior which is matched to the later forming process. The local changeof the flow behavior is achieved depending on the material, specifically

for rolled steel sheet of quality St 15 the local reduction of the flowlimit takes place by local recrystallization or recovery of the sheetmetal material,

for dual-phase steels (DP 500) the local reduction of the flow limittakes place by a local change of the martensitic and ferritic portion orby changing the martensite hardness of the martensitic phase components,

for precipitation-hardened sheet metal materials the flow limit islocally reduced by local superannuation or homogenization of the sheetmetal material.

Version B:

A local temperature change occurs during or directly prior to forming ofthe sheet metal into the shaped sheet metal piece, for example in thedeep-drawing tool or in a heating or cooling device which is connectedupstream of the deep-drawing device. In this way, in the area with thechanged temperature the flow limit is locally changed based on itstemperature dependency and the forming behavior.

Version C:

Here the sheet metal is formed in two steps. After initial deformationwith a small degree of forming, subsequent final forming takesplace—since the properties of the sheet metal to be formed have beenchanged accordingly by a local increase of temperature—with a highdegree of forming mainly in the areas in which a reduction of thematerial thicknesses is desired.

Tests which have been run on different types of steel yielded thefollowing:

Version A—local heating after one of the last rolling steps

The object of this version is the local change of the flow limit by heatwhich is added to a coil or single rolled sheet after one of the lastrolling steps, but before the sheet metal is improved. In higherstrength steels, steels ZStE 180 BH (bake hardening) and DP 500, in thetemperature range between 200° C. and 400° C., an increase of the yieldstrengths by roughly 25% relative to the initial values can beascertained. While in bake hardening steel no further strength changesoccur at higher temperatures, the values drop again in the dual phasesteel starting at temperatures above 550° C. and up to 25% of thehighest yield strength values.

In the highest strength steels, for example TRIP 800 and CP 1000, thestrength characteristics fluctuate. Relatively low strength differencesof roughly 10% can be noted overall relative to room temperaturestrength.

Result: In the steels just discussed, as per version A it is possible toadjust local yield strength changes on coils or single rolled sheetsaccording to a suitable shaped piece. Heat can be added, for example,using a laser.

Version B—Local heating directly before or during the forming The objectof this invention is the local change of the flow limit by heat which isadded to the (black) sheet directly before or during forming.

The temperature can be raised very rapidly, i.e. within seconds. In thetested steels (ZStE 180 BH, DP 500) it can be ascertained that duringheating, certain areas deform dramatically, others, in turn, onlyslightly.

In bake hardening steel the temperature must be chosen to be preferablybetween 100° C. and 200° C. in order to cause local flowability, and adecrease of strength by up to 8% compared to the initial state can beexpected. The moderate temperatures required in this case can be easilytolerated by the forming tool. In contrast, in the dual phase steeltemperatures around 200° C. or 500° C. or more are necessary, and incertain areas the local elongation can be usefully increased. Attemperatures around 200° C. a decrease of strength of at least 10%beginning at 550° C. by at least 20% compared to the initial state canoccur.

In the highest strength sheet metal qualities TRIP 800 and CP 1000 atemperature of roughly 500° C. is required to useful local elongation.Here strength decreased by roughly 22% or 28% compared to the initialstate.

Result: Version B is definitive for the bake hardening steel and can beconditionally used for the dual phase steel. Version B is less suitedfor the highest strength versions TRIP 800 and CP 1000 which requiretemperatures exceeding 500° C. to reduce the strength values. Therequired temperatures are too high for use in the forming tool. Thefollowing are possible as heat sources (temperature range: roughly 100°C. to 250° C.): oil bath, hot air blower.

For all these steel qualities the temperature range between roughly 350°C. to 450° C. seems to be irrelevant for a local change of elongation.In this area the strength is not reduced, but a strength peak which canbe substantiated with the bake hardening effect occurs.

Version C—Forming in two steps

The object of version C is forming of the sheet in two steps. After thefirst forming step it is possible to proceed as in version B.

In conjunction with the use of the process as claimed in the inventionthe following considerations should be noted.

Normally, in a simple tensile test with local heating of a tensionsample necking occurs preferably in the described area, since the yieldstrength decreases due to the increased temperature and thus an area ofespecially strong flow occurs.

The following relationships apply in the tensile tests:

σ=F/S ⁰

R _(m) =F _(max) /S ⁰

ε=ΔL/L ₀=(L−L ₀)/L ₀

Here:

σ: nominal stress

S₀: initial cross section

F: tensile force

R_(m):tensile strength

F_(max):maximum tensile force

R_(p): elongation limit, for example R_(P0.2)

ε: elongation

ΔL: extension

L₀: initial measurement length

L: respective measurement length

Since the mechanical work performed during necking is converted intoheat, the temperature continues to rise, with the result that strainhardening cannot increase to the degree that the flow stress decreasesand ultimately the sample fails. If conversely certain differences areset in the flow limit in different areas, for example 20%/10%/5%, whichcan be achieved by a staggered temperature increase, this “normal”,above described behavior can be avoided. The object of versions A, B andC is to set minor differences in the yield strengths with differentstrain-hardening behavior in the different areas.

One practical implementation can consist in that the flow limit of sheetmetal material after one of the last rolling steps is locally reduced bylocal heating in a furnace with different heating zones by means of aburner arrangement, inductive heating or by high energy radiationsources. The areas in which the strength has been reduced can berecognized by the deep-drawing press or presses by corresponding markson the surface of the sheet metal so that the sheet metal can bepositioned accordingly in the forming tool.

One practical implementation for versions B and C can consist in thatthe flow limit of the sheet metal material is changed by local heatingimmediately prior to forming in a furnace with different heating zonesby means of a burner arrangement inductively or locally by high energyradiation sources or by this taking place during forming by the actionof the corresponding heat sources.

The use of measures (for example, diode radiation) according to theinitially discussed prior art would also be conceivable.

The possibilities inherent in the process as claimed in the inventionfor creating different strengths and material thicknesses of the shapedsheet metal piece can be further expanded if the blank to be formed intothe shaped sheet metal piece is composed of for example two joined(welded) component sheets of different steel materials and/or differentsheet metal thicknesses.

In the drawings FIG. 1a shows a sheet metal blank 1 with an initialsheet thickness d₀ and with areas 1.1, 1.2, 1.3, which have been heattreated with different intensity, while FIG. 1b shows the sheet metalblank 1 formed into a shaped sheet metal piece 2 in cross section,showing that areas with different sheet metal thicknesses d1, d2 and d3also formed.

FIG. 2 shows different yield strengths with differing strain-hardeningbehavior using the two examples in FIGS. 2a and 2 b, occuring after thecorresponding heat treatment in different areas of the sheet metal blank1. Sections in which the two material areas can flow plastically arecrosshatched. This is illustrated using σ/ε diagrams.

Initial situation: In a material there are two different material states1 and 2 next to one another with different tensile strengths R_(m) ¹ andR_(m) ² and elongation limits R_(p) ¹ and R_(p) ².

In a material which has areas with different tensilestrengths/elongation limits, the following conditions must be met sothat the two material areas can flow plastically at a stipulated stresswithout a fracture occurring.

Conditions:

1) R_(p) ²<R_(p) ¹<σ

2) σ<minimum (R_(m) ² and R_(m) ¹)

for 1)

The applied stress must be greater than the higher of the two elongationlimits so that the two material areas deform plastically.

but at the same time

for 2)

must be smaller than the smaller of the two tensile strengths so thatthe material cannot fail.

In FIGS. 2a and 2 b):

□: areas of possible elongation with stipulation of a stress within theallowable area Δσ

1,2: material state 1 and 2 with R_(m) ¹ and R_(m) ² and and R_(p) ¹ andR_(p) ²

Δσ: allowable stress range in which the two material areas flowplastically without the material failing

Δε_(1,2): elongation range for material area 1 and 2 which belongs tothe allowable stress range Δσ.

What is claimed is:
 1. A process of forming a metal article of variedthicknesses comprising: effecting a temperature gradient in a metalsheet to produce at least two zones having different straincoefficients; and plastically deforming said sheet while heated toprovide correspondingly varied thicknesses of said zones.
 2. A processaccording to claim 1 wherein a selected zone to be reduced in thicknessis heated prior to plastically deforming said sheet.
 3. A processaccording to claim 1 wherein a selected zone to be reduced in thicknessis heated during plastic deformation of said sheet.
 4. A processaccording to claim 1 wherein said plastic deformation is effected bydrawing said sheet.
 5. A process according to claim 1 wherein said metalis steel.
 6. A process according to claim 1 wherein said metal sheetcomprises two joined plies of different steel materials.
 7. A processaccording to claim 1 wherein the sequences of steps are repeated.
 8. Aprocess according to claim 1 when said metal sheet is formed by rollingand is further processed, and the sequence of steps is performed betweensaid rolling and further processing.
 9. A process according to claim 5wherein the difference of temperatures between said zones is in therange of 100° C. and 200° C.
 10. A process according to claim 7 whereinsaid metal sheet is formed of a steel material, the difference intemperatures between said zones in the first sequence of steps is in therange of 100° C. and 200° C. and the difference in temperatures betweensaid zones in the second sequence of steps is in the range of 200° C.and 400° C.
 11. A process according to claim 10 wherein a selected zoneto be reduced in thickness in the second sequence of steps is heatedbetween the first and second sequence of steps.